Eye-diagram mask for optical pulses

ABSTRACT

In an optical data transmission system, high frequency pulsed optical signals are transmitted along an optical fibre. These pulses are attenuated and distorted by transmission over long optical fibres, making detection of data difficult at the receiving end. The allowable pulse profile is determined by an eye diagram, and the invention uses an eye diagram which is non-symmetrical with respect to logic 1 and 0 levels. By positioning the eye diagram closer to the logic 0 level, detection in the presence of noise introduced during transmission of the optical pulses, can be significantly improved.

[0001] This invention relates to an optical transmission system, andmore particularly in which a modulated optical signal is transmittedover an optical fibre. When a modulated optical signal is amplitudemodulated at a very high frequency, to form a succession of pulses whichrepresent information, for transmission over a long distance over anoptical fibre, the light pulses are distorted and attenuated by thetransmission process, and the degree of attenuation and distortion whichpermits recovery of the information at the far end of the optical fibredetermines the maximum distance over which the information can be sentwithout amplification. Optical amplifiers can be provided at intervalsalong the fibre, but these introduce a degradation in the optical signalto noise ratio (OSNR) due to asynchronous spontaneous emitted noise(ASE) introduced by the optical amplifiers.

[0002] If the bit error rate (BER) is too high for a particular lengthof link, the problem may simply be tackled by reducing the length ofspans between signal regeneration at which the optical signal isconverted to an electrical signal and reshaped. This causes an increasein costs because more regenerator sites are needed, and this can greatlyincrease costs and complexity for a system which transmits densewavelength division multiplex signals (DWDM) in which a single opticalfibre carries a large number of separate light channels each having adifferent wavelength (colour). Alternatively, one could provide anoptical receiver at the far end of the optical fibre having anadjustable decision threshold, but this requires modifications to thereceiver which would greatly increase cost and complexity.

[0003] The article, Gee et al, ‘Performance of an SC Duplex Transmitterfor 2.5 Gbit/s with clock and data recovery’ Electronic Components andTechnology Conference Jun. 1-4 1999, pp 201-206, IEEE, describes an eyediagram of transmitted optical pulses in which the eye mask ispositioned centrally with respect to the two logic states of thesepulses.

[0004] The present invention seeks to provide an improved opticaltransmission system. According to this invention, an opticaltransmission system includes means for generating and transmittingoptical pulses having logic one and zero levels along an optical fibre,in which the eye diagram of the optical pulses includes an eye mask,characterised in that the mean amplitude of the eye mask is closer tothe logic zero level than the logic one level, and means for receivingthe optical pulses at the far end of the optical fibre, the amplitudecharacteristic of the eye mask being arranged to facilitate detection ofthe logic one and logic zero levels at the receiving means in thepresence of noise introduced during transmission of said pulses.

[0005] Preferably the system includes means for generating andtransmitting optical pulses at a rate of 3 Gb/s or less and having logicone and logic zero levels along an optical fibre, in which the eyediagram of the optical pulses includes an eye mask which is ofrectangular shape, and which has an upper limit of 0.65 of the logic onelevel, and a lower limit of 0.15 of the logic zero level.

[0006] In practice, the standard nominal bit rate is 2.5 Gb/s, which isan actual bit rate 2.48832 Gb/s, but it may be desirable to incorporate,for example, forward error correction, and this requires the insertionof additional bits into the bit stream which increases the optical pulserate above the nominal value of 2.5 Gb/s. The figure of 3 Gb/s isselected so as to allow for an increase in the bit rate above itsnominal, standard, value.

[0007] Preferably the eye mask is a rectangle having a time positioncentred on the peak amplitude position of an optical pulse. Preferablyagain, the length (duration) of the eye mask is 0.2 of the eye diagrambit interval.

[0008] The pulses received at the far end of the optical fibre, arepreferably converted to an electrical signal and detected by thresholddecision means which is a.c. coupled to a preceding conversion means.This results in a mean pulse level which is determined by the pulseprofile.

[0009] The invention is further described by way of example withreference to the accompanying diagrams in which:

[0010]FIG. 1 illustrates an optical transmission system,

[0011]FIG. 2 shows part of the system in more detail, and

[0012]FIGS. 3 and 4 are explanatory diagrams.

[0013] Referring to FIG. 1, there is shown therein an n-channel DWDMoptical transmission system. The system consists of n optical channels 1which are generated at individual optical transmitters 2. Each opticalchannel is a light signal which is modulated with traffic and overheadinformation, and in a DWDM system a large number n of optical channelsare provided with each channel having a different optical carrierwavelength. The adjacent wavelengths can be closely spaced, and may bespaced regularly apart over the available spectrum.

[0014] The n-channels are combined at a multiplexer 3, where theindividual carrier wavelengths are preserved, and transmitted via abooster amplifier 4 over an optical fibre 5 to a pre-amplifier 6 of areceiver. The optical fibre may be part of a very long haul transmissionsystem eg of the order of 600 km, and to boost the signal level optionaloptical line amplifiers 7, 8 may be provided as necessary.

[0015] The received optical signal is fed to a demultiplexer 9, whereeach of the individual optical carriers is separated out and passed toan individual receiver 10, where the traffic and overhead informationcan be recovered and used as required.

[0016] Each optical carrier has a very high frequency, and can transmitdata as a string of light pulses at a very high repetition rate, eg at2.48832 Gb/s (ie the nominal 2.5 Gb/s rate) or higher. The actual bitrate of the light pulses can be higher than the nominal value ifadditional error correction bits are inserted into the bit stream forthe purpose of error correction. One forward error correction techniqueincreases the bit rate by the ratio 15/14, but alternative techniquesmay cause a greater or lesser increase with a value of 3 Gb/s providingan expected upper limit. At bit rates of this order, the shape of thesepulses is modified and degraded during transmission along the opticalfibre 5. This degradation can be caused by the optical fibre itself, andby the optical amplifiers used which generate asynchronous spontaneousemission of noise which adversely affects the signal to noise ratio.Each of these effects gives rise to a noisy signal and an increased biterror rate at the receiver.

[0017] In order to reduce the bit error rate, it has been proposed toreduce the lengths of spans between amplification, and to reduce thenumber of spans before regeneration which involves anoptical-to-electrical signal conversion. This has the disadvantage of anincrease in the costs of the system, and more regenerator sites arerequired, and the cost penalty of regeneration in DWDM applications witha large number of channels can be severe.

[0018] Alternatively, in order to reduce the bit error rate, thedecision threshold level at the receiver may be adaptable to produce anoptimum bit error rate. Considerable modifications would be needed atthe optical receivers to adjust the threshold level to allow fordifferent values of input power.

[0019] Instead the invention utilises a transmitted optical modulationand a receiver having a fixed decision characteristic selected withregard to the characteristics of the transmitted pulses.

[0020]FIG. 2 shows part of the receiver in more detail. Only a singlechannel receiver RX is shown, but the arrangement would be replicatedfor all optical channels. The light at the input of the receiver RX isdetected by an optical-to-electrical detector 20, and the resultingpulsed signal is fed via a fixed gain trans-impedance amplifier 21 todecision 4 circuitry 22 via a capacitor 23 which a.c. couples thecircuitry 22 to the detector 20 and amplifier 21.

[0021] The pulse shape of the optical signals transmitted by thetransmitter is shown in FIG. 3, which illustrates a so-called eyediagram, in which pulse amplitude is plotted against time. A logic 1pulse shape 30 is shown superimposed on a logic 0 pulse shape 31, and itwill be seen that the pulse shapes are asymmetrical with respect to theamplitude mid-point 32, that is to say there is a reduction in the dutycycle of the pulses. The permissible position of the pulse shapes isdefined by the rectangular eye mask 33, which is offset from themid-point 32, and this rectangle 33 determines the limit of the shape ofthe pulses which are transmitted, as neither a logic 1 pulse or a logic0 pulse may intrude on the area defined by the rectangle.

[0022] If the nominal logic 0 value is zero, and the nominal logic 1value is unity, the upper level of the rectangle has a value of 0.65 andthe lower level a value of 0.15. The duration of the eye diagram is T,which corresponds to a single pulse length. The duration of the eye maskis 0.2 T.

[0023] In FIG. 2, the amplifier 21 is capacitively a.c. coupled to thedecision circuit 22 by the capacitor 23, and the electrical signal whichis obtained from the optical-to-electrical signal conversion is biasedat the input of the decision circuit 22 around the threshold level—thismeans that the threshold level and the mean level of the incoming signalare the same.

[0024] The duty cycle y of the optical signal is$\gamma = \frac{t_{1}}{2T}$

[0025] where t₁ is the time during which the signal is above its meanlevel and T is the bit interval, as shown in FIG. 3.

[0026] In an optical transmission signal the probability of sending alogic one generally equals the probability of sending a logic zerobecause the signal is scrambled before transmission in order to maintain0.5 the probability of each symbol. A signal profile in accordance withthe invention is shown in FIG. 4, in which, for the above conditions,the mean value V_(M) of the signal is that for which the two areas 40,41 are equal.

[0027] To maintain equal the two areas when γ is lower than 50%, ΔV₁must become greater than ΔV₀. If the electrical signal is biased acrossthe decision level that distinguishes logical ones from logical zeros(which is commonly the case in AC coupled receivers of which that shownin FIG. 2 is an example) the logical ones are moved further away fromthe threshold level V_(M) by altering the characteristic of thetransmitted signal and without the need to modify the receiver.

[0028] This way of biasing the electrical signal is auto adaptive and itis not affected by variation of the input optical power because the meanlevel of the signal does not change (the signal is AC coupled) and evenif the peak to peak amplitude changes the ratio ΔV₁/ΔV₀ is constantbecause it is related to the value of γ (a parameter of the transmitter,independent on the received power).

1. An optical transmission system including means (TX) for generating and transmitting optical pulses having logic one and zero levels along an optical fibre, in which the eye diagram of the optical pulses includes an eye mask, characterised in that the mean amplitude of the eye mask (33) is closer to the logic zero level than the logic one level, and means (RX) for receiving the optical pulses at the far end of the optical fibre, the amplitude characteristic of the eye mask (33) being arranged to facilitate detection of the logic one and logic zero levels at the receiving means in the presence of noise introduced during transmission of said pulses.
 2. An optical transmission system as claimed in claim 1 including means for generating and transmitting optical pulses at a rate of 3 Gb/s or less and having logic one and logic zero levels along an optical fibre, in which the eye diagram of the optical pulses includes an eye mask which is of rectangular shape, and which has an upper limit of 0.65 of the logic one level, and a lower limit of 0.15 of the logic zero level.
 3. An optical system as claimed in claim 1 or 2 and wherein the eye mask is a rectangle having a time position centred on the peak amplitude position of an optical pulse.
 4. An optical system as claimed in claim 3 and wherein the length (duration) of the eye mask is 0.2 of the eye diagram bit interval.
 5. An optical system as claimed in any of the preceding claims and wherein the pulses received at the far end of the optical fibre are converted to an electrical signal and detected by threshold decision means which is a.c. coupled to a preceding conversion means. 